There is continuously increasing a transmission capacity of the communication systems, and the capacity has been significantly increasing due to a progress of an optical fiber used in a communication system. For example, the communication systems are categorized into a point-to-point trunk line network system, a metro network system and an access network system, the optical communication system has already been deployed in the trunk line network system, in addition, even in the metro network system and the access network system, an electrical communication system is going to be replaced with the optical communication system. Namely, the whole communication system is progressing to adopt the optical communication system.
Recently, the information capacity transmitted via one optical fiber has been dramatically increased by use of a wavelength division multiplexing system. In this system, when using a low loss band width at 0.4 bit/Hz spectrum transfer efficiency, the transmission capacity per one optical fiber becomes approximately 3.2 Tbit/s. Concretely, this capacity can be realized by 320 channels based on the 10 Gbit/s transmission speed per channel (wavelength) which is available today.
It is inevitable for an optical signal to deteriorate in its waveform, timing and intensity when it travels a long distance. As such, the optical signal which has traveled a certain distance is inevitably required to be regenerated. Therefore, a regenerating system is normally provided in the optical communication systems so that the degenerated signal light is recovered. This regenerating system typically comprises: a receiver unit which receives a degenerated signal light and converts it into an electrical signal; a regenerator unit which performs a necessary function such as amplifying, noise reduction, waveform reshaping, and clock signal regeneration for the electrical signal; and a transmitter unit which reconverts the regenerated electrical signal to the optical signal then sends out to the optical carrier.
Concretely, in the optical signal regenerating system which is inserted in the middle point of the optical fiber wherein a plurality of light signals in 320 channels travel, 320 units of receivers, regenerators and transmitters are equipped in order to accommodate the number of channels.
There has been a difficulty in downsizing and a disadvantage of large power consumption in the optical signal regenerating system which includes so many units.
Furthermore, when the optical signal regenerating system is used in a metro network system and an access network system, it is necessary to install 320 channel optical signal regenerating systems at many relay points, and consequently the whole optical communication system becomes bigger and the cost and the power consumption increase.
As one of the solutions for the problems, there is an approach of reducing the number of channels in the wavelength division multiplexing system and increasing its transmission speed of each channel. This is because, the transmission speed is inversely proportional to the number of channels in the wavelength division multiplexing system when maintaining the same transmission capacity.
However, the regenerator used in the optical signal regenerating system is made of an electric device to process the electric signals so it has a physical upper limitation in the response speed. For example, a maximum signal transmission speed processed in an electric device is 40 Gbits/s today. Further, at this speed, still 80 channels are needed. Furthermore, when an electronic device is driven at higher speed, the higher electric power is needed. Therefore, the regenerating system using an electric device has a limitation in the transmission speed of each channel and it is difficult to realize a downsizing and reduce the power consumption in the real application.
As another possible solution for the problems with an optical signal regenerating system using an electronic device, there is a method of using whole optical signal regenerator which processes on the optical light itself without converting it to an electric signal. The equipment of the whole optical signal regenerator typically comprises a high speed electrical modulator and a regenerator utilizing a nonlinear optical effect of the substance.
However, this whole optical signal regenerator uses an electric modulator, so that there is an upper limitation in the processing speed similarly to the optical signal regeneration system which performs a photo-electro conversion. Further, when regenerating an optical signal of 40 Gbits/s or faster transmission speed, the transmission speed of the optical signal is reduced by a time division process, and after regenerating the signal, a time division multiplexing is required and hence it becomes a large system.
Because the whole optical signal regenerator uses a nonlinear optical response (nonlinear optical effect) for regenerating an optical signal, the following problems are caused.
Namely, in a large capacity optical communication system, when the light of a wavelength division multiplexed signal which includes a plural signal lights with a different wave length transmits through the optical transmitting path, the polarizing condition of each optical signal becomes slightly different among each wavelength after transmitted. If the regenerating process is performed at once by a whole optical signal regenerator using a nonlinear optical effect, the intensity of the nonlinear optical effect varies depending on the polarization condition of the incident light, and consequently the wavelength division multiplexed light after regenerated may include components of the light that is not well regenerated or sometimes no components regenerated at all.
In the above, the problems are pointed out focusing the polarization condition of the incident transmitting light in the optical regenerator. In the below, the today's problems will be discussed focusing on the optical regeneration system. The optical regeneration means re-amplification of a light intensity, re-shaping of a waveform, and re-timing of an optical signal which is degenerated during a transmission (hereinafter, abbreviated to O3R). The optical regeneration system which has those functions can transmit a signal light to the infinite distance (long distance) of the optical fiber.
Regarding the infinite distance (long distance) transmission described above, it is reported by Leuthold et al. (Leuthold et al., Electron. Lett,. 38, p 890, 2002). This paper describes about 40 Gb/s, 1,000,000 km transmission using an O3R regenerator.
In this paper, Leuthold et al used an electronic circuit technique for the optical clock extraction required for re-timing (a technique of generating a clock pulse train synchronizing with the transmitting optical signal) and for the switching function. As such, this system cannot satisfy the transmission speed which is limited by the electrical circuit technique. For example, it is not applicable to 160 Gbits/s system which exceeds the limit of the speed of the electronic circuit.
Note that regarding the 160 Gbits/s system, it is reported about the optical regenerator adopting an optical switch. (Schubert et al., Electron. Lett,. 38, p 903, 2002). However, it is anticipated not to work as an O3R device because the clock extractor is not provided.
Summarizing the above, the O3R has not been yet realized which fundamentally utilizes an effective whole light technique. However, there are many reports about a method of re-shaping of the waveform and re-timing of the clock which are one of components in the O3R. It will be demonstrated below but limited to the technique of an optical fiber which fundamentally relates to the present invention.
At first, the whole optical waveform re-shaping method will be described. This can be generally categorized into two methods. First one is a method utilizing a solution which is a combination effect made by a nonlinear optical characteristic and an anomalous dispersion of an optical fiber (Hasegawa and Tappert, Appl. Phys. Lett., 23, P 142, 1973). The waveform re-shaping device based on this technique is called as a solution converter. Second one is a method based on the self phase modulation effect of a light pulse utilizing a nonlinear characteristic of an optical fiber (Mamyshev, ECOC' 98, p. 475, 1998). This is called as a Mamyshev filter originated from a name of a proponent. A method utilizing a supercontinuum light is positioned as a follow-up model.
As an experiment using the former system (solution converter), the transmission of 4×40 Gb/s signal for 10,000 km is reported (Dany et al., Opt. Lett., 25, p. 793, 2000). The latter system (Mamyshev filter) is used in the 1,000,000 km transmission line in the Leuthold's system (Leuthold et al., Electron. Lett,. 38, p 890, 2002) as described above. All of them are reported from a view point of systems but has not described clearly about the performance of the device itself (only a few report such as Dany et al., ECOC' 01, We. P. 45, 2001). Especially, in the solution converter, because a noise amplification by a solution effect becomes a problem (Kubota et al., J. Opt. Soc. Am B, 16, p 2223, 1999), the designing from view point of a device performance is important as well. Namely, identifying the device performance clearly and establishing an optimum design rule of the device itself will be a future subject to be solved.
Next, a method of re-timing method will be described. The main stream of this technique is a complex method of an optical clock extraction and an optical switching. FIG. 38 shows this configuration. It includes an optical clock extraction part and an optical switching part. The former (an optical clock extraction part) is to synchronize the phase of the input signal (an optical signal or an electrical signal which corresponds to a carrier signal) and the local optical light (an optical pulse train which functions as a standard clock, called an optical local oscillator (abbreviated to optical LO). Namely, an optical clock extraction is realized by synchronization between the input optical light and the local optical light. Therefore, the optical LO is required to have a variable characteristic of the repetitive frequency.
The latter (an optical switching part) is an optical switch such as a four wave mixing (FWM) device or a nonlinear optical loop mirror (NOLM), utilizing a nonlinear effect of an optical fiber which can realize a multiplication function in the optical region. The details of the optical clock extraction and optical switch technique will be described below.
In FIG. 38, the area enclosed by a dotted line is an optical clock extraction part which comprises an optical phase comparator, an optical LO generator and a controller circuit. The optical phase comparator senses the phase difference between the external optical signal and the optical LO, and controls the optical LO oscillating frequency (which corresponds to the repetitive frequency of the pulse train) to minimize the error.
Consequently, a synchronization between the external optical signal and the optical LO is realized to produce a precise time position pulse train (hereinafter, called a clock pulse train) which is synchronized with the external optical signal. By utilizing a nonlinear optical effect instead of an electronic circuit technique, a phase controller which is capable of working at speed of exceeding 160 GHz can be realized. The method of a synchronization using a phase comparator in an optical region as described above is called an optical phase locked loop (OPLL). Actually, there has been proposed a clock extractor which is carried out by a synchronization between a beat light and an external optical signal from two LDs using a NOLM (Bigo et al., U.S. Pat. No. 6,239,893 B1).
Remarkable point is a timing jitter (time fluctuation of a clock pulse) of the output optical clock train of the OPLL. A timing jitter means a shift of the clock pulse in the time scale. Since the jitter can be a cause of deteriorating the performance in the communication systems, its suppression is important. The timing jitter of the clock pulse train has a correlation with the OPLL operation speed and when the OPLL operation is getting faster, the jitter is reducing. Namely, increasing an operation speed of OPLL is effective for jitter reduction.
However, when a NOLM as described in above technique (Bigo et al., U.S. Pat. No. 6,239,893 B1) is used, the OPLL loop length becomes a longer size, and hence the band width of the OPLL operation is limited. As a result, it restricts a reduction of jitter. To solve this, shortening a fiber length of the optical nonlinear device used for the phase controller will become a key factor. By shortening the fiber length, the OPLL operation band width can be free from the restriction, Namely, it can realize a fast speed OPLL operation and generate a high quality clock pulse train with less jitter.
There is required an optical switch technology for the re-timing along with the clock extraction technique described above. A method of using an FWM which is a typical example of an optical switch utilizing an optical fiber nonlinear effect will be described below. When two chromatic lights with different wavelengths enter an optical fiber, newly colored optical lights different from the original color are generated if either of the incident optical lights has a sufficient power for the nonlinear effect. This is an FWM phenomenon.
When the clock pulse train and the optical signal enter into the optical fiber, the FWM light includes an imposed information of the input optical signal and its pulse timing is determined by the clock pulse train. Consequently, the optical signal pulse train with low jitter in which the information is imposed can be obtained. This is a principle of re-timing based on FWM. However, not only dispersion effect but also nonlinear effect give an influence to the optical pulse traveling in the optical fiber.
By this effect or the combined effect of those effects, the pulse waveform changes during the pulse traveling. As a result, the waveform distortion of FWM light is generated. It is effective to optimize a fiber dispersion value or the input power in order to suppress this, but the real value or the control method have not been disclosed.
The minimum components required for the optical regeneration system are described above. In addition, a method to improve the performance of the optical regeneration system is also important. Two components relating to the present invention are summarized below. Namely, (1) a device to convert an optical pulse waveform to a wave shape suitable to the optical switch and (2) a device to isolate a component of the optical pulse from a component of optical noise.
Firstly, the rectangular pulse waveform method which is effective to suppress the increased noise intensity in the optical switch will be described concerning (1) a device to convert an optical pulse waveform to a wave shape suitable to the optical switch. Typically in the optical switch, as a result of an interaction between the nonlinear optical characteristic and the dispersion, the time jitter of the input of a transmitting optical signal pulse is converted to the intensity jitter in the output signal after regenerated. FIG. 39A is an explanatory view.
Here, an optical switch for a pulse train including a jitter and a clock pulse train will be discussed.
The output pulse power from the optical switch correlates with a time overlapping between a transmitting pulse and a clock pulse. As a result, a change in the overlapping region between both pulses caused by a jitter is converted to an output pulse power of the optical switch. In order to suppress the amount converted from the phase jitter to the light intensity jitter, a rectangular clock pulse waveform conversion on the transmitting optical signal pulse or the extracted clock pulse is effective (FIG. 39B).
This rectangular method is categorized into a method utilizing a chromatic dispersion or a polarization mode dispersion, and a method utilizing a complex effect of nonlinear effect and a normal dispersion. The former example is a method using a fiber Bragg grating or a polarization maintaining fiber (Lee et al., OFC2001, PD30-1, 2001 and Schubert et al., Electron. Lett,. 38, p 903, 2002), and the latter example is a method using a normal dispersion fiber (the principle reported, Nakatsuka et al., Phys. Rev. Lett., 47, p. 910, 1981).
In the former method based on the linear function, a steepness of the rising edge and falling trail of the rectangular waveform before transforming is determined by the input pulse width. Namely, in order to obtain a steep slope of the rectangular pulse, the very narrow optical pulse is needed accordingly. Compared with that, the latter case has an advantage of ability to transform to a steep slope rectangular wave, however, the higher power of an input optical light and a longer fiber length are the mandatory required to obtain nonlinear effect and dispersion effect.
Secondly, a device to isolate a component of optical pulse from a component of optical noise (2) will be described. In the below, the noise reduction component is summarized. The optical pulse has a noise imposed thereon. The major component of the noise is an amplified spontaneous emission light (ASE) generated when amplifying an optical light. In general, a noise has a wider optical spectrum than an optical signal, and hence the noise components which are out of the optical signal bandwidth can be eliminated by the optical filter to some extent.
However, the noise components within the bandwidth of the optical signal remain. To eliminate those noise components, there is a proposed method utilizing an optical solution characteristic, besides a method using a waveform reshaping described above. Here, attention is directed to the latter case which relates to the present invention.
There is reported about phenomena (solution self frequency shift: SSFS) that the solution shifts towards a longer wavelength side by a stimulated Raman scattering (SRS) in the optical solution transmission (Mitschke and Mollenauer, Opt. Lett., 11, p. 659, 1986). This phenomenon also happens when the ASE noise is added to the solution. It is proposed a method of noise reduction utilizing this characteristic which isolates the solution from ASE noise components to filter the same in the frequency domain by using this phenomenon (Namiki et al., Provisional Publication No. 2001-109024). The configuration of the noise reduction unit is shown in FIG. 40A.
It comprises an anomalous dispersion fiber (ADF) and an optical filter. An optical solution including noise components shown in FIG. 40B upper part is inputted to the ADF. During the transmission, SSFS of the optical solution component is carried out by the SRS. Further, the remarkable point is that the solution components shift towards longer wavelength side by the SRS but the noise components stay without shifting towards the longer wavelength side. Consequently, the noise components within a bandwidth of the optical signal can be eliminated by a method of extracting the solution components by the output optical filter after shifted. (FIG. 40B lower part).
Furthermore, this phenomenon has a function of shifting the wavelength as well, and therefore it is possible to adjust the wavelength of the optical signal to the desired wavelength by SSFS control. However, SSFS is a phenomenon which typically occurs in the femto second region, so that further improvements for the pico second solution transmission is necessary in the SSFS and its efficiency.
As described above, today's status and problems about the optical regeneration systems are detailed. The present invention is aiming to solve these problems and provide simple O3R systems.
The purpose of the present invention is to solve the above described problems and to provide a wavelength division multiplex optical regeneration system and a wavelength division multiplexing optical regeneration method which is capable of realizing a large transmission capacity, a small footprint and a power saving.